Wavelength-Dependent Singlet Oxygen Generation in Luminescent Lanthanide Complexes with a Pyridine-Bis(Carboxamide)-Terthiophene Sensitizer

Article abstract of DOI:10.1002/chem.202000587

Lanthanide ion (Ln^III) complexes, [Ln(3Tcbx)₂]³⁺ (Ln^III=Yb^III, Nd^III, Er^III) are isolated with a new pyridine-bis(carboxamide)-based ligand with a 2,2′:5′,2′′-terthiophene pendant (3TCbx), and their resulting photophysical properties are explored. Upon excitation of the complexes at 490 nm, only Ln^III emission is observed with efficiencies of 0.29 % at 976 nm for Ln^III=Yb^III and 0.16 % at 1053 nm for Ln^III=Nd^III. Er^III emission is observed but weak. Upon excitation at 400 nm, concurrent ¹O₂ formation is seen, with efficiencies of 11 % for the Yb^III and Nd^III complexes and 13 % for the Er^III complex. Owing to the concurrent generation of ¹O₂, as expected, the efficiency of metal-centered emission decreases to 0.02 % for Yb^III and 0.05 % for Nd^III. The ability to control ¹O₂ generation through the excitation wavelength indicates that the incorporation of 2,2′:5′,2′′-terthiophene results in access to multiple sensitization pathways. These energy pathways are unraveled through transient absorption spectroscopy.

Full text of DOI:10.1002/chem.202000587

A Journal of
Accepted Article
Title: Wavelength-dependent singlet oxygen generation in luminescent
lanthanide complexes with a pyridine-bis(carboxamide)-
terthiophene sensitizer
Authors: Ana de Bettencourt-Dias, Katherine R. Johnson, Sebastian B.
Vittardi, Manuel A. Gracia-Nava, and Jeffrey J. Rack
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of the final Version of Record (VoR). This work is currently citable by
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202000587
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Supported by

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Wavelength-dependent singlet oxygen generation in luminescent
lanthanide complexes with a pyridine-bis(carboxamide)-
terthiophene sensitizer
[
a]
[b]
[a]
[b]
Katherine R. Johnson, Sebastian B. Vittardi, Manuel A. Gracia-Nava, Jeffrey J. Rack,* and Ana
de Bettencourt-Dias* [a]
[
a]
K. R. Johnson, Dr. M.A. Gracia-Nava, and Dr. rer. nat. A. de Bettencourt-Dias
Department of Chemistry,
University of Nevada, Reno
Reno, NV, 89557, USA
E-mail: abd@unr.edu
[
b]
S. B. Vittardi, Dr. J. J. Rack
Department of Chemistry and Chemical Biology
University of New Mexico
Albuquerque, NM, 87131, USA
Email: jrack@unm,edu
Supporting information for this article is given via a link at the end of the document.
III
3+
III
III
Abstract: Lanthanide ion (Ln ) complexes, [Ln(3Tcbx)
2
]
(Ln = Yb ,
many of the examples cited, the effectiveness of these systems
strongly depends on the nature of the coordinated metal, as it
promotes intersystem crossing (ISC) through the heavy atom
III
III
Nd , Er ), were isolated with a new pyridine-bis(carboxamide)-based
ligand with a 2,2′:5′,2′′-terthiophene pendant and their resulting
photophysical properties were explored. Upon exciting the complexes
effect.[14] Other effective 1O
photosensitizers are based on
nanoparticles, nanosheets, or metal-organic frameworks.
2
III
[1a, f, h, 15]
at 490 nm, only Ln emission is observed with efficiencies of 0.29%
III
III
III
III
III
at 976 nm for Ln = Yb and 0.16% at 1053 nm for Ln = Nd . Er
However, these compounds often require elaborate syntheses,
numerous purification steps, and frequently suffer from low
solubility and poor long-term stability. Fullerene is a unique
example of a photosensitizer, as it shows high efficiencies of ISC,
despite the absence of heavy atoms, and a 1O2 of 1. Its surface
can be functionalized in attempt to increase compound solubility
or add other functionalities, but functionalization disrupts the
fullerene core, decreasing 1O2.[16] Despite these drawbacks,
small molecule-based systems are actively pursued due to the
ability to tune their properties through further functionalization.
Many of the compounds mentioned above are fluorescent and
amenable to in situ and in vivo tracking. In practice, background
auto-fluorescence and high concentrations of these compounds
emission is observed but weak. When exciting at 400 nm, concurrent
1
III
III
O
2
formation is seen, with efficiencies of 11% for the Yb and Nd
III
complexes and 13% for the Er complex. Due to the concurrent
1
generation of
2
O , as would be expected, the efficiency of metal-
III
III
centred emission decreases to 0.02% for Yb and 0.05% for Nd . The
1
ability to control
2
O generation through the excitation wavelength
indicates that incorporating 2,2’:5’,2”-terthiophene resulted in access
to multiple sensitization pathways. These energy pathways were
unravelled through transient absorption spectroscopy.
Introduction
prevent useful imaging.[17] Lanthanide ion (LnIII) luminescence is
highly desirable for imaging due to characteristic emission
wavelengths, large pseudo-Stokes shifts of sensitized emission,
and long decay lifetimes. The long lifetimes enable time-gated
detection of the emission, which leads to spectra with large signal-
to-noise ratios.[18] The metal-centred light emission of the LnIII ions
is based on transitions within the 4f orbitals. These f-f transitions
are parity-forbidden, which poses a challenge for direct excitation.
Instead, coordinated ligands are used to absorb light and transfer
energy to the LnIII ion to achieve LnIII-centred emission. More
1
Compounds with the ability to generate singlet oxygen ( O
2
)
are useful as cancer therapeutics in photodynamic therapy
[2]
pesticides,[3]
(
PDT),[1] antimicrobial drugs and antibiotics,
[
4]
sterilization and decontamination agents, and in oxygenation of
natural products, among other applications.[6] 1
[
5]
O is cytotoxic
2
because it oxidizes a variety of biomolecules, such as membrane-
bound mitochondrial proteins, DNA, and lipids.[2b, 7] The most
1
studied photosensitizers for O
2
generation in situ are porphyrins
and related tetrapyrroles,[2b, 6] whereas hematoporphyrin and
other variants are frequently used for PDT.[1c, 8] Effective doses of
porphyrins for biological applications are limited by dark toxicity,
specifically, the ligand absorbs energy to populate a ligand-based
singlet excited state, S. Subsequent ISC generates a ligand-
based triplet excited state, T. Energy transfer from the T state to
[
9]
cutaneous photosensitivity, and stability due to dimer formation.
III
the Ln -based f* excited state results in metal-centred
Poor solubility[ and aggregation
excited state, resulting in low efficiencies of 1O
1O2).[12] BODIPY is another frequently used photosensitizer. Its
10]
[11]
shorten the lifetime of the
generation
luminescence. This sensitization process is referred to as the
2
[18]
antenna effect.
(

The use of organic ligands as antennae is advantageous, as
they can be appropriately modified to tailor specific properties and
yield compounds with dual properties. Recently, the use of rare
earth complexes with dual properties in cancer diagnosis and
derivatives have high extinction coefficients in the visible region
of the spectrum, do not suffer as strongly from photobleaching,
and are synthetically easier to handle.[1d, 13] Despite high 1O2 for
1
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Chemistry - A European Journal
10.1002/chem.202000587
FULL PAPER
therapeutics was reviewed by Termini and Gray.[19] Long and
Stasiuk[20] described several compounds that are responsive and
highly stable in vivo. This provides real-time bio-tracking and
bioimaging and offers promising opportunities for advancing
research.
called 3Tcbx, and its luminescent Ln complexes (LnIII = YbIII,
III
III
III
Nd , Er ). Depending on the excitation wavelength, these
complexes show only Ln -centred emission or concurrent Ln
emission and 1
generation. While these complexes are not
III
III
O
2
water-soluble and cannot be used for imaging and therapy
Examples of LnIII complexes with dual properties include
purposes, they are the first example of wavelength dependence
1
generating LnIII complexes. Thus, they
magnetic resonance imaging (MRI) contrast agents, which
for luminescent and O
2
concurrently sense oxygen,[21] and complexes that generate O
1
,
provide design principles for the development of new compounds
that can be used for therapy and imaging.
2
which can simultaneously be tracked through MRI[22] or EuIII-
based emission.[23] For example, porphyrin-based Ln complexes
emit in the near-infrared (NIR) with 1O2 in the range 1.5% to
III
24]
III
4
.0%,[ and a Tb complex with naphthyl and azaxanthonyl
Results and Discussion
III
Tb
functional groups display Tb -centred emission efficiency ( ) of
4% and 1O2 = 12%.
pyrene-appended Yb complexes, which display metal-centred
[
25]
2
Faulkner and co-workers describe
3
Tcbx, shown in Figure 1, was prepared by Pd-catalyzed
Suzuki-Miyaura cross-coupling between 4-bromopyridine-2,6-
III
emission with
atmospheric O
a
strong dependence on concentrations of
dicarboxamide and the boronic acid of 2,2′:5′,2′′-terthiophene
III
2
; however, Yb emission quenching is observed
upon the formation of 1O .[26] Patra and co-workers describe
(
4
Figure S1). After purification, the compound was recovered in
2% yield and was characterized using standard techniques (see
2
luminescent LnIII complexes of DTPA-bisamide with quinoline
pendant as a photosensitizer. The complexes show red and green
Supporting Information for details).
In acetonitrile, 3Tcbx absorbs in the UV-Vis region (Figure S2)
with an absorption maximum of 395 nm, and has a molar
III
III
1
Eu - and Tb -centred emission. Concurrent formation of O
2
was
1
inferred through the use of O
2
traps and given the cytotoxicity of
-1
-1
absorptivity () of 42,150 M cm . This value is similar in
the complexes.[27] Maury and co-workers report a tris-picolinato-
,4-7-triazacyclononane macrocycle with 1O2 = 80% when
[36]
magnitude to other aryl-functionalized oligothiophenes.
This
1
compound emits in the visible region of the spectrum with a
maximum at 496 nm (Figure S3) and a fluorescence quantum
III
III
complexed to Gd . However, when Yb is coordinated, only
metal-centred emission is observed.[28] The same authors also
isolate helicene-based complexes, which display efficient 1
yield (
F
) of 9% (Table 1). This value is similar to other
can range from 2% to
emission at 1270 nm (Figure S4) shows that
generator. We determined 1O2 = 60% (Table 1),
O
2
oligothienyl-based systems, where 
F
generation, with 1O2 as high as 91% and concurrent (but weak)
[37]
1
65%.
The
O
2
Ln -centred emission. More recently, Zhang et al.[30] report a
III
[29]
1
3Tcbx is a O
2
III
double-decker Yb -porphyrin complex with a metal-centred
which is comparable to other aryl-substituted oligothiophenes,
with reported 1O2 ranging from 23% to 80%,[35, 38] and to known
III
emission efficiency of ~3% in water. While the Yb complex does
1
III
not generate O
2
, its Gd analogues do with 66% efficiency.
PDT photosensitizers, such as clorin-e
6
(64% in phosphate
III
There are also examples of Ln -based materials with dual
properties. These include Nd -doped nanoparticles (NPs) with
surface appended chlorin-e6 that when combined generate O
buffer),[39] chlorophyll a (44% in ethanol),[40] riboflavin (48% in
III
methanol), porfimer sodium (89% in phosphate buffer),[39] and
[41]
1
I
I
I
[
4
2
]
2
L
u
t
e
x
a
p
h
y
r
i
n
(
2
3
%
i
n
m
e
t
h
a
n
o
l
)
.
[
31]
emission leading to in vivo tumour reduction.
Upconverting
Ln -doped NPs decorated with Rose Bengal also display 1O
III
2
generation.[32] A NP doped with Yb , Er , and Tm was decorated
with a 1
-generating bacteriochlorin derivative and an orange
emitting 1,8-naphthalimide derivative; it could be selectively
III
III
III
O
2
[
33]
excited in the NIR for safer tracking. In addition, a NP with an
III
III
III
Yb /Tm -doped-core and a Nd -containing shell was explored for
its functionality as a 1
generator, and as an optical contrast
agent and nanothermometer, using NIR luminescence.
A limitation of many of the light-emitting photosensitizers
described above is that they luminesce and generate 1O
O
2
[
34]
2
simultaneously, limiting their potential as a therapeutic because
they do not offer the exploratory convenience of tracking the
cytotoxic compounds without generating the reactive oxygen
species. This can result in unintended harm to cells/tissues in their
path when attempting to track such compounds. To overcome
this, we aimed to develop dually-functional luminescent
compounds that can only luminesce at specific excitation
wavelengths to make it possible for luminescence tracking without
the risk of harming tissues. With this goal, we targeted the
]3+ in acetonitrile at 25.0 ± 0.1 C (LnIII
2
Figure 1. Emission spectra of [Ln(3Tcbx)
Nd (purple), Yb (teal), Er (pink)). 1
III
III
III
=
O
2
emission at 1270 nm is shown for
III
Nd only, for clarity, but is observed at 1270 nm for all complexes. The structure
of 3Tcbx is shown.
functional group 2,2′:5′,2′′-terthiophene, which is a biologically
1
active floral pigment and a well-studied O
reported in the range 60% - 86%.[35]
2
generator with 1O2
LnIII complexes were isolated by mixing 3Tcbx with Ln(NO
)
3 3
We report here the synthesis, characterization, and
photophysical properties of a 2,2′:5′,2′′-terthiophene-derivatized
N,N,N',N'-tetraethyl-pyridine-2,6-dicarboxamide photosensitizer,
III
III
III
III
(
Ln = Nd , Gd , Er , and Yb ) in acetonitrile, in which the
complexes display good solubility. The correct stoichiometry of
the complexes in solution was determined through absorption
2
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Chemistry - A European Journal
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FULL PAPER
titration of the YbIII or NdIII nitrate salts with 3Tcbx (Figures S5a &
To assess the ability of 3Tcbx to act as a sensitizer of LnIII
luminescence, the energies of its S and T states as the
S6a). The resulting stability constants are 9.19 ± 0.05 (log

),
III
[Gd(3Tcbx)
2
]3+ complex were determined in degassed n-hexanes
17.99 ± 0.07 (log

), and 23.13 ± 0.18 (log

) for Nd
[
43]
complexation and 9.26 ± 0.01 (log

), 17.89 ± 0.01 (log

), and
(Figures S7 & S8). The S and T levels are at 22,700 and 21,300
cm-1 (Table 1), respectively, and thus, this ligand is only able to
III
23.00 ± 0.22 (log

) for Yb complexation (Table S1). The
sensitize the NIR-emitting LnIII (Nd , Yb , and Er ). The energy
III
III
III
speciation diagram (Figures S5b & S6b) showed the 2:1 species,
3
+
III
[
Ln(3Tcbx)
2
] , as the dominant species, which was thus used for
levels are too low to sensitize the visible-emitting Ln ions.
photophysical characterization.
Table 1. Singlet (S) and triplet (T) excited state energies, quantum yields of fluorescence ( ), sensitized luminescence (Ln), and 1
F
O
2
generation (1O2) of 3Tcbx
3+
III
III
III
III
III
and its metal complexes ([Ln(3Tcbx)
Tcbx
F
2
]
where Ln = Gd , Yb , Nd , or Er ) in acetonitrile at 25.0 ± 0.1 ºC.
3
GdIII
YbIII
NdIII
ErIII

1O2
S
T
F

1O2
Yb

1O2
Nd

1O2
Er

1O2
-
1
-1
[
%]
[%]
[cm ]
[cm ]
[%]
[%]
[%]
[%]
[%]
[%]
[%]
[%]


exc= 400 nm
exc= 490 nm
9 ± 1
[c]
60 ± 6
[b]
13 ± 2
[c]
62 ± 1
[b]
0.02 ± 0.01
0.29 ± 0.03
11 ± 1
[b]
0.05 ± 0.00
0.16 ± 0.04
11 ± 3
[b]
[a]
[a]
13 ± 3
[b]
2
±
2,700
70
21,300
±10
[
a] = emission was too weak to allow determination of . [b] = emission was not observed. [c] = was not determined.
Upon excitation of the complexes at their absorption
maximum (400 nm), the characteristic metal-centred emission is
0.02%). A similar increase, from 0.05% to 0.16%, is observed for
III
III
the Nd complex. The Er emission efficiency was again, not
observed (Figure 1). For [Yb(3Tcbx)
2
] , we observed F5/2  2F7/2
3
+
2
quantifiable. The emission efficiencies obtained for the Nd and
III
3
+
YbIII complexes are on the same order of magnitude to values
reported for other complexes of these ions.[45]
at 976 nm, and in the case of [Nd(3Tcbx)
2
] , we observed the
(J = 9/2 – 13/2), with the most intense transition at 1053
4F3/2  4I
J
4
4
3+
nm. We also observed I13/2  I15/2 at 1513 nm for [Er(3Tcbx)
2
] .
To understand the unusual photophysical behaviour of these
complexes, we investigated the possibility of multiple ligand-
based excited states by ultrafast transient absorption
III
Ln
Quantum yields of sensitized Ln emission ( ) are summarized
3+
in Table 1. [Yb(3Tcbx)
2
]3+ has a Yb = 0.02% and [Nd(3Tcbx)
2
]
Nd
III
]3+ as a representative
has a  = 0.05%. These values are comparable to other Yb
spectroscopy. We focused on [Nd(3Tcbx)
species in this class (Figure 3), though we also investigated
2
III
[44]
III
and Nd complexes. The emission intensity of the Er complex
3
+
III
III
III
III
is too low to determine the efficiency reliably.
[Ln(3Tcbx)
2
] , where Ln = Yb , Er , or Gd (Figure S10).
]3+ at 430 nm produces a transient
In addition to sensitizing LnIII emission, these complexes
Excitation of [Nd(3Tcbx)
2
generate 1
O
2
, as evidenced by the emission peak at 1270 nm
featuring a large excited state absorption (esa) at  > 550 nm and
what appears to be two overlapping bleach (negative absorption)
features with peaks near 400 nm and 475 nm (Figures 3a & 3b).
The higher energy bleach matches well with the ground state
3
+
3+
(
2 2
Figure 1). 1O2 are 11% for [Yb(3Tcbx) ] and [Nd(3Tcbx) ] and
1
3% for [Er(3Tcbx)
2
]3+ (Table 1). The largest 1O2 is 62% for the
Gd complex, indicating that coordinating luminescent Ln (NdIII,
III
III
Yb , or Er ) to 3Tcbx decreases 1O2 due to sensitization of LnIII
luminescence, as would be expected and has been observed in
other Ln complexes that display both properties.[28] Nonetheless,
III
III
0
absorption and is thus ascribed to loss of the S ground state. The
-1
longer wavelength bleach peak (~21,000 cm ) does not
correspond to a ground state absorption and is tentatively
III
the values of
photosensitizers, such as the ones indicated above.
Additional photophysical characterization

1O2 are comparable to values for known
assigned to stimulated emission from S
1
, which is in good
-
agreement with the steady state emission results (S at 22,700 cm
1
of
these
, Table 1). As time evolves, one observes a narrowing of the esa
compounds revealed, interestingly, that the excitation maxima
obtained when monitoring LnIII emission (NdIII at 1053 nm, YbIII at
9
at 655 nm. This occurs with a time constant of 0.54 ps. There are
slight concomitant spectral changes in the bleach region on this
timescale, indicating similar dynamics for electron and hole.
Following this kinetic phase, the 655 nm esa merges to a broad
absorption ranging from 450 to 650 nm, while the longer
wavelength bleach (475 nm) diminishes in intensity to yield a
single broad esa. The bleach at 400 nm and broad spectral esa
persist throughout the duration of the experiment (~7000 ps),
indicating a long-lived excited state.
Single wavelength kinetics at multiple points (475, 665, and
730 nm) reveal three conserved time constants of 0.54 ps, 68 ps,
and 322 ps (Figure S11). These kinetic components will be
discussed later within the framework of the electronic structure of
2,2′:5′,2′′-terthiophene.
76 nm, and ErIII at 1513 nm) are all red-shifted by ~45 nm in
comparison to the excitation spectra obtained when monitoring
1
3
the O
2
2
 O transition at 1270 nm (Figure 2a). As hoped for,
when measuring the emission spectra of all luminescent
compounds using the lower energy excitation, we did not observe
1
3+
O
2
emission. For example, when [Nd(3Tcbx)
2
1
]
is excited at 400
2
emission (Figure
III
nm, we observe both Nd luminescence and O
2
observe only Nd emission (Figure 2b, green). A similar
behaviour is also observed for YbIII (Figure S9), although the
b, black); however, when [Nd(3Tcbx)
2
]3+ is excited at 490 nm, we
III
metal-centred
phosphorescence peak.
Ln were determined as well at 490 nm for [Ln(3Tcbx)
are summarized in Table 1. Upon excitation at 490 nm,
Yb(3Tcbx)
]3+ emits with an efficiency of 0.29%, more efficient
when compared to the excitation at higher energy (exc = 400 nm,
emission
intensity
dwarfs
the
1O
2
]3+ and
While excitation at 430 nm yielded a long-lived transient,
excitation at 470 nm does not yield any appreciable transient
beyond 4000 ps (Figure 3d). Despite this difference, there are
some notable similarities. The initial transient features an esa at
2
[
2
3
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Chemistry - A European Journal
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FULL PAPER
6
50 nm reminiscent of that produced from 430 nm excitation.
excitation, these data provide compelling evidence that the
formation of different excited states are factors of excitation
wavelength. In accord with the emission data, we note that the
femtosecond transient absorption data (temporal and spectral
However, there appears to be only a single bleach peak near 420
nm.
This bleach feature mirrors the ground state absorption
(
Figure 3, dashed line). There is no discernible negative peak
response) with 470 nm excitation for [Yb(3Tcbx)
similar to that described above for [Nd(3Tcbx)
]3+ (Figure S10).
3
]3+ are very
near 475 nm, and thus no evidence for stimulated emission.
Kinetic analysis at 735 nm retrieves three separate time constants
of 0.31 ps, 3.2 ps, and 485 ps (Figure S11), which are assigned
based on literature data of 2,2′:5′,2′′-terthiophene.[46] In
comparison to the pump-probe data collected from 430 nm
3
These observations are critical in elucidating operative decay
pathways within the excited state profile.
(a)
(b)
3
+
1
emission or LnIII emission (LnIII = YbIII, Nd , Er , or Gd ). (b) Excitation (dot-dashed)
+
]3 in acetonitrile at 25.0 ± 0.1 C. Excitation spectra were measured at 1053 nm (green) and 1270 nm (black) and
2
III
III
III
Figure 2. (a) Excitation spectra of [Ln(3Tcbx)
and emission (solid) spectra of [Nd(3Tcbx)
2
]
while monitoring at either O
2
emission spectra were collected with excitation wavelengths of 400 nm (black) and 490 nm (green).
4
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Chemistry - A European Journal
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FULL PAPER
Figure 3. Femtosecond pump-probe spectroscopy of [Nd(3Tcbx)2]3+. Transient spectra of probe delays between 0.3 ps and 10 ps at excitation wavelengths 430
nm and 470 nm (a and b, respectively). Spectral traces of delays between 70 ps and 2500 ps at excitation wavelengths 430 nm and 470 nm (c and d, respectively).
Dashed traces represent the ground-state absorption spectrum.
2
,2′:5′,2′′-Terthiophene features wavelength dependent
excited state behaviour and is capable of sensitizing 3O .[46-47]
Experimental and theoretical studies converge on a model that
invokes an S state above S and two T states. For 2,2′:5′,2′′-
terthiophene, the ISC mechanism is contingent upon excitation
energy, whereby access to the T state is facilitated by a rapid
transfers energy to a Nd* state that emits on a much longer
timescale (s). It appears that while T is of sufficient energy to
produce 1
, it more efficiently undergoes energy transfer to
produce Nd* and coordinated ligand S , thus yielding no
absorption change in the transient experiment.
2
1
O
2
n
0
n
0
2
ISC channel embedded higher in energy within the singlet
manifold. Pump-probe data also reveal kinetic components
associated with torsional motion between thiophene rings.[48] Our
transient absorption data of the 3Tcbx ligand are consistent with
those reported for 2,2′:5′,2′′-terthiophene (Figure S12). The
spectroscopic data of [Nd(3Tcbx)
2
]3+ show strong similarity with
both 3Tcbx and 2,2′:5′,2′′-terthiophene.
We propose the following electronic structural model to
3
+
describe the dynamics of the [Ln(3Tcbx)
High energy excitation leads to the formation of an S
rapidly relaxes to the state. Consistent with previous
studies,[48a] internal conversion from T
2
]
complexes (Figure 4).
1
* state that
T
2
2
to T
1
is inefficient. The
3
long-lived T
2
state is suitable for bimolecular reactivity with O
. We note that the free ligand also produces O
2
1
1
yielding O
2
2
upon
excitation, similar to 2,2′:5′,2′′-terthiophene. Lower energy
excitation induces an S population near the excited state minima,
which intersystem crosses to T . The T state subsequently
1
1
1
5
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Figure 4. Excited-state model for [Ln(3Tcbx)
2
]3+
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4
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Conclusion
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containing the O
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-generating moiety 2,2′:5′,2′′-terthiophene. We
III
1
2
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1
exciting at high and low energy. On the other hand, cytotoxic O
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applications where
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simultaneous tracking of the complexes by Ln -centred
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The National Science Foundation (CHE 1800392 to AdBD and
CHE 1602240 to JJR) is gratefully acknowledged for financial
support. Prof. Jorge H.S.K. Monteiro (Humboldt State University)
is thanked for his assistance with absorption titrations and some
emission spectroscopy.
Keywords: 2,2′:5′,2′′-terthiophene • dual functionality •
luminescence • singlet oxygen • wavelength dependence
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Entry for the Table of Contents
1
Wavelength-dependent O
2
generation in NIR-emitting lanthanide ion complexes with 2,2′:5′,2′′-terthiophene-based sensitizers.
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